Frequency indicating system



i ing the frequency of Patented Dec. 26, 1944 UNlreo I STATES d PATEN'l t FFICE 2,366,076 FREQUENCY'INDICATING SYSTEM York application August 19, 1941, serial No 407,503

2 Claims.

This invention relates to frequency indicai'nrs` l and in particular to arrangements for measurl periodic variations of electric currents.

The frequency indicator of the present invention is of the electronic" type employing vacuum tubes. Electronic frequency indicators havealready been devised which employ thyraj tron#l grid-glow" and other types of gaseous discharge tubes, sometimes referred to as trigger" type of tubes. Such indicators depend for their action upon the periodic starting and stopping of the gaseous discharge through the tube under the control of the current whose frequency is to be measured. 'I'llese prior frequency measuring devices are'not entirely satisfactory due to the fact that thyratron and other gaseous types of tubes deteriorate rather rapidly when used at high frequency, and their characteristics alter markedly and necessitate continual compensation. Furthermore, in the use of such trigger type of devices involving a gaseous discharge, it is necessary `to recondition or restore the tube after each conducting operation so the tube will be in condition for the next cycle of operation.

'I'he present invention involves4 an. electronic frequency indicator which employs vacuum tubes of the high-vacuum, pure-electron discharge type having characteristics which remain con stant over long periods of time and are unaffected by the frequency of the signals applied to them. Furthermore, such tubes do not require the resetting or restoring operation which is essential to the yoperation of tubes of the thyratron or trigger type.

'Ihe present invention also permits a straight line or proportional relation between the instrument reading and the -applied signal 'frequency, or ifdesired a rising or falling or other- Wise varied characteristic may be obtained.

Generally stated, the invention involves a vacuum tube oscillator which normally is in Aa non-oscillatory state but is caused to execute one complete oscillation per cycle cf applied signal or periodic current. The amplitude of this oscillation, once started, is independent of the amplitude of the controlling signal and causes a unidirectional pulse or charge to flow through an indicating meter. Thus the total charge Aper second, or the average value' of the current through the meter is dependent upon the frequency of the controlling signal and is used as an indication of that frequenc If the unidirectional pulses or charges are uniform, the meter indication will be in direct` proportion to the frequency of the applied signal or current. The Vcharges can, however, be made to vary-gradually as the frequency varies, in which case the characteristic of the meter is modiiled. The invention will now be `described by reference to the accompanying drawing in which Figure 1 is a. diagram illustrating the basic circuit to which the invention is applied;

Figure 2 is a diagram showing the circuit embodying the invention. This circuit is characterized by high stability when powered from a source of alternating current and includes means for varying the characteristics of the indication with reference to the frequency of the .applied signal; l

Fig. 3 is a. curve illustrating variations in the direct current component of the plate current of controlled oscillator of Fig. 2; Fig. 4 illustrates pulses produced by the varying current illustrated in Fig. 3.

In the arrangement shown in Figure 1, the periodic current to be measured is supplied to the input circuit which includes a condenser I' and a resistance 2 connected in series. The voltage variations produced across resistance 2 are applied between the cathode and the grid of tube 3. The plate circuit of tube 3 includes a coupling resistance 4 and a suitable source of plate current indicated by the battery il. A coupling condenser 5 and a resistance 6 are provided for transmitting to the grid of tube 'i current pulses developed by current changes inthe plate circuit of tube 3. A battery i2 is included in circuit with resistance 6 for normally maintaining the grid of the tube 'l sufficiently negative with respect to the cathode of this tube to prevent flow of current through tube 'I when a static condition obtains.

ance 8. A condenser 9 provides a suitable backcoupling from the plate element of tube 'i to the grid element of tube 3. A suitable indicating when a negative potential is applied by the signal l through condenser i and across resistor 2 `to the Plate current for tubel 4'i issupplied from battery il through a resistgrid of vacuum tube 3, this causes a reduction of current through the tube and consequently through potential of tube 3. 'I'he increased plate potential causes a charging current to flow to condenser. through resistor 6, and the potential drop across resistor 6 acts to decrease the-negative potential applied to the grid of tube 1 from battery I2. In other words, a negative impulse applied to the grid of tube 3 causes a decrease in the negative potential applied to the grid of tube 1. 'Ihis results in a ilow of current through tube 1 and resisto;` B thereby reducing the potential on the plate of tube 1 which causes condenser 9 to discharge through resistor 2. The potential drop across resistor 2 applies a negative potential to the grid of tube 3. This negative potential' produces the same action in tube 3 and in the remainder of the, circuit as the potential originally applied from the -input circuit, continuing the action initiated by the input signal. This action continues independently. of the existing value of the applied input signal, decreasing the current in tube 3 and increasing the current in tube ,1 until further changeiin the currents of these two tubes is prevented by the operating characteristics of the tubes and associated circuits. As the current in tube 3 decreases towards its limiting value, the charging of condenser 5 through resistor 6 decreasesand thereby decreases the potential on the grid of tube 1 which causes a decrease in -the current in this-tube and thereby initiates a reverse operation which continues until the plate current in tube 1 is reduced to zero. Battery I2 Vprevents current flow in tube 1 until another signal of proper polarity is applied to tube 3 from the input circuit. Thus, a unidirectional pulse or charge is passed through meter I0, the amount of lthis charge being determined by the operating characteristics of thecircuit and not by the'amplitude of the input Iresistor 4, thereby increasing the plate.

alternating current source. The biasing current for tube 1 is supplied from rectifier I2a which may be energized from the same alternating current source that supplies rectifier IIa. The output of rectier Iza is bridged by a circuit including a potentiometer resistance I2b connected in series with a two-electrode glow discharge tube I2c. A second potentiometer resistance I2d is connected at one endto the variable tap on potentiometer I2b and the other end is connected to shunt tube I2c as shown. The lead for supplying biasing current to tube 1 through resistor 6b is connected to the variable tap on potentiometer For the purpose of producing a greater current through meter I0, a pair of additional vacuum tubes 1a and 1b are connected with their cathodes and grid elements arranged in parallel with the cathode and grid element of tube 1, the plate signal. -This action will be repeated for eachv l negative pulse of input signal applied to the grid of tube 3 and thus the'total charge vper second, or the average current passing through meter I0 is dependent uponthe frequency of the applied signal. Any averaging type ofcurrent meter may be used for meter I0 and its reading may be used as a measure `o f the frequency of the applied signal. I

Figure 2 is a circuit diagram showing the invention, and is an improvement on the circuit arrangement shown in Figure 1 designed to give high stability when energizedv 'from a source-of alternating current. Various elements shown in Figure 2 which correspond tov similar elements.

shown in Figure 1 are represented by the same reference numerals. A's will be seen, the basic pulsing circuit is represented bythe elemen 2-3-4-5-6af-Bb-6c-1--3-9-Il In this arrangemen-tthe` element corresponding to resistance 3 in Figure l is replaced by two resistance elements 6a andjbfconnected on opposite sides of a rectifier Sc--included in sei-ies in the connection between condenser 3 and the grid -of tube 1. A high resistance l8d is included between the rectifier 6c and tube .1 to prevent appreciable lowering'of the input impedance of tube 1 incase the input potential should become large enough to cause electron ilow from the grid of tube 1 or from the grids.. of the tubeswhich have theirgrid-cathode circuits'connected in parallel with that of tube 1.

^ The plate currents for tubes 3 and 1 are supplied by rectier vI la which 1S energie@ .fmm

elements of these additional tubes being connected in parallel' and supplied with plate current from rectifier IIa, as shown. These additional tubes are'not essential to the operation of the vcircuit but merely provide an amplified indication.

For purposes indicated hereinafter, a resistance I0a is connected in series with the indicating meter I0, and a pair of condensers Illb and Ilc are connected in shunt with meter I0 and resistance Illa, as shown,. a variable resistance Illd being included in series with condenser IIlc. A condenser 6e is also connected between a variable tap on resistance 6b and the lower terminal of resistance 6b. y

An oscillating circuit is interposed in the 'input circuit of vacuum tube 3, and .this oscillating circuit comprises a vacuum tube. 'I3 of the pentode type having a cathode, three grids a, b, and c, I

and an anode d. The current whose periodicity is to be measured is supplied between the cathode and grid Vc by means of a circuit including a condenser Ia and a resistance I3a. Plate current for tube I3 is supplied from rectier Ila through a circuit which includes the primary of transformer T, a feed-back coil I3b and an inductance coil I3c, the negativeterminal of rectifier IIa being connected to the common cathode lead.

A tuned oscillatory circuit consisting of an inductance coil i3d and a condenser I3e connected in parallel is included in the circuit to grid a,

and this grid circuit also includes a high resistance I3i shunted by acondenser'l3y. Feedback coil I3b is inductively coupled to inductance. coil i3d to produce high frequency oscillations inthe circuit, the circuit being tuned to a frequency which preferably is much higher than the frequency to be measured. A grid b of tube I3 is supplied with biasing potential from rectifier I la, as shown. Resistance I3a connected in series` .with grid c is sufdciently large to prevent appreure 2 is as follows:l

When a signal applied to the input circuit reduces the negative potential on grid c, the oscildue to electron current f change in current is very rapid the pulse will be independent of the frequency of by the action of inductance coil Itc. A negative potential is developed on grid a during oscillation through the network of resistor |3f and condenser potential causes the total electron, current through the tube to decrease rapidly to a low value. This total current passes through grid b and plate d and thus through the primary of transformer T. Condenser |3h by-passes the high frequency oscillatory currents around transformer T. The form of the current through the primary of transformer T is shown in Fig. 3, where it will -be seen that the current has a relatively high value in the non-oscillatory state and suddenly reduces to a lower value when the condition of oscillation is established, as shown at the point A. This sudden change in the current through transformer T induces a pulse in the secondary of transformer T, and this pulse is passed through a frequency consisting of condenser lb, resistor 2b and re sistor 2c,deve1oping across resistor 2c a Apulse of potential as shown at A' in Fig. 4. In a similar manner, when the input signal ceases and allows the oscillator to changefrom an oscilla-4 tory state to a non-oscillatory state, the total current through transformer T will suddenly increase to its former value as shown at B in Fig. 3,r and this sudden increase in current will result in establishing a current pulse across resistance 2c of the same shape as pulse A' but of opposite polarity, as shown at B in Fig. 4.

Thus, the pulse across resistance 2c will be of one polarity for a change from a non-oscillatory state to an oscillatory state and of an opposite polarity for the reverse change. Since the form of the the input signal and may be made shorter duration than one complete input signal.

The pulses of negative polarity only are passed through rectifier tube 2d and` thence developed across resistor 2 and applied to the grid of tube 3, thereby producing an amplified positive potential pulse across resistor 4. This positive pulse is transmittedby means of condenser 5, resistor 6a, rectifier 6c and resistor 6b through resistor d to the grids of tubes 1, 'la and 1b. The grids of tubes 1, 'la and 1b are normally l maintained at a negative potential suilicient in,

value to prevent. a ow of electron current through them, but the transmitted positive pulse permits electron current to flow through tubes l, 'la and .1b and thus through resistor' Illa and meter IU. The electron current flowing through tube 1 -develops an amplified negative potential pulse across resistor t and this negative pulse is transmitted through condenser 9 to resistor 2 and thence to the gridof tube 3. Since a negative potential pulse applied to grid of tube 3 started the described'events, the pulse is selfmaintained and grows 'in size untillimited by the characteristics of the tubes used and their asl sociated circuit elements.

After the pulse reaches a limiting size the reverse actiontakes place automatically and the pulse decreases until the original normal stable condition is reached. Therefore, each cycle of input signal causes a given charge, to flow through meter lil, and the'total chargev per second, or the average Value of the current through meter selective network cycle o'f the l I0 is dependent upon the frequency of the applied signal. l

Rectifier 2d prevents any reaction of the selfmaintained pulse upon the apparatus initiating Powder supply devices such as lla and |2a are vin common use and usually involve a rectifier and filter assembly. It is well known that the cutput potentials of such devices vary as the potential applied to them from the alternating current mains varies. 'Ihis is true even though a so-called regulated power supply device is used as is the case of power supply l'la. Therefore, if the potential delivered by the alternating current mains increases, the potential delivered by power supply Il a increases, as does also the temperature of the cathodes of the Vvacuum tubes when they are powered fromthe same alternating current source. For a given frequency of input signal these effects by themselves tend to increase the size of the current pulses through meter I 0, thereby giving an erroneous reading. However, an increase of the negative lbiasing potential applied to the grids of tubes 1, 1a and 1b produces the reverse effect, i. e. decreases the size of the current pulses through meter Il). The potential drop across tube |2c remains substantially constant even though the current through it varies in accordance with the variations in potential of the output of power supply |2a caused by the varying potential supplied .by the alternating current mains. Since resistor I2d`is connected across tube l2c and a fraction of resistor l2b, the potential across resistor I2d may be made to vary in the same proportion as the fpensate for any variation in reading of meter I0 dueto a Variation of to a variation in output cathode temperature' or potential of power supply Ila caused by variation in potential of the alternating current supply or to both these effects. In actual practice it was found that for a given frequency of input signal the reading of meter I0 could be kept constant to within 0.2% for a variation ofA. C. main supply potential from volts to 130 volts. These figures areonly illustrative o1' the possible stabilizing action.

The size of `current pulse per cycle through meter l0 may be varied as a function of the frequency ofthe applied signal by means of condenser 6e and resistor lod and condenser'ic. Condenser 6e increases the size of current pulse as the frequency increases. Decreasing the size r of resistor ld tends to decrease the size of the current pulse as the frequency increases. In creasing the value of either condenser lb or Illc will decrease the size of the pulse. Thus the frequency response characteristic may be made to bend up, to be straight, `to bend down, or to be a composite oi'these eects depending upon the relative values of the componentsused.

Electronic frequency indicating systems embodying the inventive concept disclosed herein require no conditioning before operation since no current will flowthrough the indicating meter signal is applied. The frequency-response characteristic may be varied to nt specialized work and the reading is substantially independent of fluctuations of potential of the alternating current supply when such a supply is used as a primary source oi.' power.

1. In a frequency measuring system, the combination of an electron current type tube including a cathode, an anode and at least one grid, an anode circuit yfor said tube including a source of current, a control circuit for said tube, meansl for coupling said anode circuit and control circuit to produce self-sustained high-frequency oscillations therein, means responsive to said bination of a vacuum tube oscillator having a grid circuit and an anode circuit coupled to saidV grid circuit for producing oscillations, Ameans controlled by said Voscillations for reducing the anode current of said oscillator, means for biasing the grid of said tube to normally prevent oscillations for reducing the -value of anode current flowing in said anode circuit, meansfor bi-v asing the grid of said tube to normally prevent oscillations, a source of periodic current whose frequency is to be measured, means for applying said periodic currents to said grid to cause said tube to oscillate for a time during each' cycle of said currents, means responsive to changes in said anode current for derivinga uniform current pulse forV each current change, and means controlled by said pulses for producing-an indication proportional to the time-integrated value of said pulses.

2. In a frequency measuring system, the `comj the current iiowins oscillations. a source of periodic current, means for applying said periodic current to said grid to cause saidtube to oscillate for a time-during each cycle of said current, means responsive to changes in anode current of said tube for pro' ducing a uniform current pulse for each change thereof, an amplifier tube having grid and anode circuits, means for biasing the gridof said am-` pliner tube to normally prevent the flow of anode current, connections for supplying said current pulses to the grid of said amplifier tube to produce an'anode current pulse for each positive pulse applied thereto, an amplifying feedback connection from the anode circuit or said amplifying -tube to the grid circuit thereof for amplifying said anode current pulses to a predetermined invariable amount, and an indicating meter connected in the anode circuit of said amplifying tube for indicating the average value of therein.

DONALD A. WILBUR. 

